During the operation of semiconductor integrated circuits (ICs), some of the electrical power carried by the IC is converted into heat. This is particularly the case with high power ICs. Thus, it is important to be able to monitor the temperature of an IC; particularly those implemented using CMOS designs. When a device's operation is temperature sensitive, a temperature reduction process may be employed.
At higher temperatures it is known that the IC's characteristics change and reliability decreases. Furthermore, under certain conditions, for example in the event of power overload, short-circuit of connections, external heating of the IC, etc., the excess heat can lead to an undesirable ‘over-temperature’ condition. Thus, in order to detect the over-temperature condition, and in order to protect the semiconductor ICs, an IC is often provided with a temperature protection circuit or device.
One example of a temperature-dependent IC is a lamp driver and associated switch, say to drive an incandescent lamp, where the heavy load current may heat the IC driver (and/or switch) excessively. With regard to the switch used in such circuits, it is known that bulky and expensive electromechanical relay switches have a relatively high failure rate. Such relay switches have therefore been superseded by integrated silicon switches that are much smaller (i.e. they can be implemented on an integrated circuit). Advantageously, such integrated silicon switches have a significantly lower failure rate; say in the order of ten times less.
In this regard, and referring first to FIG. 1, a known device's operation is illustrated graphically 100. In FIG. 1, the graph 100 illustrates how a bulb voltage varies, i.e. a gate-source voltage Vgs (in Volts (V)) 115, in relation to a drain-source current Ids (in Amps (A)) 105 and the voltage applied to the bulb 150, i.e. a drain-source voltage Vds (in Volts (V)) 110.
A first mode illustrates an ‘open’ circuit 165, where the switch is in an ‘OFF’ position. Advantageously, for the voltages illustrated, this results in an off-phase power dissipation of ‘0’ Watts, as no current reaches the bulb 160.
A second mode illustrates a ‘closed’ circuit 155, where the switch is in functional and in an ‘ON’ position. For the same voltages illustrated a closed circuit 155 results in an on-phase power dissipation of the bulb 150, for example dissipating only ‘1’ Watt.
However, as illustrated in the graph 200 of FIG. 2, the inventors have also identified a failure mode that is of particular concern for power switches. FIG. 2 shows the electrical characteristic of a failed power switch device particularly highlighting a failure mechanism that might exist: a resistive short circuit 255. The metal oxide semiconductor field effect transistor (MOSFET) device acts like a resistance with no control, i.e. the switch no longer operates as a switch since it cannot be placed in an ‘ON’ or ‘OFF’ mode of operation.
For the voltages illustrated, the bulb 250 is turned on at a Vds of 100 mV with a 10 A Ids current, and is turned off with a 12V drain-source voltage with a nominal Ids. With a semi-short circuit 255, the gate-source voltage Vgs follows a linear operation 205. However, this results in an on-phase power dissipation of the bulb 250, for example of the order of ‘30’ Watts. Such power dissipation causes a significant and damaging effect on the driver and switch, as well as the load, i.e. the bulb in this case. In particular the switch suffers from excessive amounts of heat.
Typically, when a predetermined ‘maximum’ operating temperature threshold is exceeded, a controller is arranged to shut down the power switch, or at least reduce the power supplied, thereby preventing a significant and potentially damaging increase in temperature within the power switch. This process is often referred to as the ‘shutdown/restart’ principle. Once the IC has cooled down, for example by, say, 10° C., the IC may be switched ‘ON’ again.
In order to implement a ‘shutdown/restart’ operation, temperature sensors, which are typically configured as resistors, diodes or transistor sensors, are by necessity integrated in close proximity to the corresponding hottest point in the power switch.
There exist a number of techniques for on-chip temperature sensing. One technique includes the use of a pair of on-chip thermally responsive diodes, coupled to an off-chip current source. The diode pair generates a differential voltage output that is proportional to temperature. This technique for sensing on-chip temperatures requires numerous connections between the IC and external circuitry for each temperature sensing circuit.
Small, self contained on-chip temperature sensors have a much lower cost than sensors requiring connections to circuitry external to the IC. External sensors are those sensors that are not located on the IC itself. These sensors do not provide real-time results. Furthermore, the sensors are unable to measure the circuit temperature at the location on the IC of the highest power dissipating circuit.
U.S. Pat. No. 6,323,531 B1 describes a two-chip power IC, in which a sensor chip comprises a sensor and is mounted on a switch chip having a switch. The sensor is electrically connected to the switch in order to turn the switch ‘off’ when a temperature detected by the sensor exceeds a threshold value.
European patent application EP 0 262 530 A1 describes a configuration comprising a power IC and a control circuit, which is integrated in a semiconductor chip. The power IC, together with the control circuit, is thereby mounted on one of the surfaces of the semiconductor chip.
Thus, the only known prior art in addressing excess operating temperatures of ICs, or their associated die, attempts to monitor the temperature at the hottest point of the device or the die. Consequently, a need exists to improve the reliability of an integrated circuit, for example one comprising a power device such as a silicon switch, relating to problems as a result of excess temperature effects, such as temperature gradient.